The gas leak at the Aliso Canyon Storage Facility — plugged last week after nearly four months — was an unparalleled disaster. Though the cost of stopping the leak and relocating thousands of families is still being calculated, Southern California Gas has estimated that it could run as high as $300 million.

But that figure does not include the most far-reaching impact of the leak: the tremendous effect on the climate from releasing 94,000 metric tons of methane into the air. The leak generated emissions equivalent to adding 1.6 million cars to the road.

It is, of course, impossible to undo the damage to the atmosphere. But Gov. Jerry Brown has ordered the gas company to "fully mitigate" the climate effect of the leak. It is not clear what this will mean, or what it will cost. As environmental groups prepare their arguments over the issue, there are few, if any, precedents to rely on.

"We're charting some new territory," says Tim O'Connor of the Environmental Defense Fund.

Polluters are generally required to clean up their messes. But when cleanup is not possible, some attempt is made to offset environmental effects. The most common example is wetland development, which occasionally is permitted on the condition that other land is set aside for conservation. But it's relatively new to apply the concept to unlawful carbon emissions.

"We can't just let them write a check and be done with it." —Tim O'Connor, Environmental Defense Fund

"It's not as if there are dozens and dozens of cases that establish a clear road map," says Frank O'Donnell, president of Clean Air Watch.

His group is urging the Environmental Protection Agency to impose stringent offset requirements on Volkswagen for cheating on emissions tests. He pointed to similar penalties that were imposed on Kia and Hyundai for emissions violations.

But the auto industry is covered by clear pollution-control standards. Natural gas storage is not regulated under California's cap-and-trade system, which sets a limit on pollution and allows emitters to trade carbon credits on an exchange. That makes the Aliso Canyon leak a more difficult case.

At the heart of the debate is the issue of how to put a price on carbon. Some argue that this is impossible, so the state should instead focus on levying stiff fines and enacting new regulations.

But others contend that there is a fair way to do it, which would both punish the gas company and result in real benefits for the environment. One idea is to require the gas company to buy carbon credits from the state cap-and-trade exchange, which would lower the state's pollution cap.

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"I think that's the cleanest way of ensuring the atmosphere is made whole," says Alex Jackson, an attorney at the Natural Resources Defense Council, who suggests the proceeds from the purchase could go toward pollution-reduction projects.

Depending on how the figures are calculated, the cost could run to $50 million or more. For comparison, Hyundai and Kia were forced to surrender credits worth $200 million for overstating their cars' fuel economy.

Some environmentalists think allowing SoCalGas to buy credits would be letting the company off too easy.

O'Connor argues that the release of methane ought to be offset by the retention of methane — not by a reduction in carbon dioxide. He estimates that Southern California Gas loses 300,000 tons of methane every year via small leaks in the course of normal operations — three times the amount lost at Aliso Canyon. "There are opportunities across the value chain for us to be finding these reductions," he says. "That's where we need to be focusing."

His goal is to avoid an argument over how to convert methane emissions into equivalent amounts of carbon dioxide.

Methane is a powerful greenhouse gas, though it does not linger in the atmosphere as long as carbon dioxide. Over a 20-year horizon, it's about 87 times more potent than carbon dioxide. But over 100 years, it's more like 25 to 36 times more potent.

O'Connor worries that if the state gets drawn into a debate over which numbers to use, it will end up underestimating the climate impact of the leak.

Food & Water Watch also does not believe the gas company should be able to use carbon credits. Scott Edwards, an attorney with the organization, argues that carbon brokers scour the country looking for climate mitigation projects that are already in place, which can be used to offset pollution in California.

The California Air Resources Board, which administers the cap-and-trade program, has been given until March 31 to come up with a mitigation plan for Aliso Canyon. Dave Clegern, a spokesman for CARB, says it would be unworkable to require the gas company to buy carbon credits.

"Cap-and-trade requires a fair amount of certainty since it's based on a market," he tells the Weekly via email. "One emergency can't be allowed to throw the market out of balance."

Last fall, CARB issued a draft strategy to reduce emission of "short-lived" pollutants such as methane, black carbon and hydrofluorocarbons. Among the ideas are the removal of wood-burning fireplaces from homes; increased composting to reduce methane emissions from landfills; and better monitoring of industrial gas leaks.

"We'd be open to the mitigation of this leak simply being folded into that larger strategy," the NRDC's Jackson says.

The short-lived pollutant strategy is likely to form the starting point for CARB's discussion over the Aliso Canyon response. CARB is expected to take up the issue for the first time at a hearing on Feb. 18.

Rob Jackson, a Stanford professor who has studied methane leaks, suggests that SoCalGas should be forced to spend money on monitoring and research that would lead to industrywide upgrades.

"It all starts with measuring and monitoring leaks," Jackson says.

Gas utilities have already undertaken costly improvements to their distribution systems in response to the San Bruno explosion, which killed eight people in 2010. Whatever measures are imposed on Southern California Gas, environmental groups agree that it should be in addition to existing requirements.

L.A. elected officials also have weighed in on the debate, urging Gov. Brown to focus the mitigation efforts in the community around Porter Ranch. According to Alexandra Nagy, an organizer with Food & Water Watch, Mayor Eric Garcetti has talked about using mitigation money to buy electric buses.

But for Edwards, all of this discussion is somewhat beside the point. Mitigation, he argues, is akin to a license to continue to pollute.

"We can't mitigate what's already been done. The methane genie is out of the bottle in Porter Ranch," Edwards says. "We believe this facility should be shut down and the company should be heavily fined. That money should be going to promoting clean, renewable energy so we can get off fossil fuels sooner rather than later."

For its part, the gas company has committed to full mitigation but has not offered a proposal on how to do that.

"Right now, the company's top priorities are to permanently seal the well and support the community as they begin to return home," says Javier Mendoza, a company spokesman.